DESCRIPTION (Applicant's abstract): The goal of this research is to elucidate the anatomical and physiological interactions among muscle fibers, motor nerves, and the local control of capillary perfusion. The thin, parallel-fibered hamster retractor muscle has been developed as a model system for studying these interactions using video microscopy and image analysis. When muscle fibers are active, oxygen requirements increase along their entire length, which encompasses many microvascular units (MVUs) of capillary flow control. However, the anatomical correspondence among MVUs which supply a fiber or fiber bundle is not understood. Therefore, AIM I is to define the organization and perfusion of MVUs with respect to muscle fiber position and contractile activity. Through imaging microvessels and muscle fibers, we will test the hypotheses that (i) MVUs align with muscle fibers; and that (ii) MVUs supplying a fiber (bundle) originate from common parent arterioles. Stimulation with microelectrodes will be used to test the hypothesis that (iii) muscle fiber contraction results in perfusion of MVUs along the active fibers. Acetylcholine (ACh) triggers vasodilation that is conducted rapidly along the arteriolar wall to increase MVU perfusion. Because neuromuscular junctions (NMJs) are the source of ACh in muscle, AIM II is to define the spatial and functional relationships among NMJs and MVUs. By labeling and imaging NMJs and MVUs, we will test the hypothesis that (i) motor innervation corresponds with microvascular topology. Stimulating NMJs while observing blood flow in microvessels will be used to test the hypotheses that perfusion of MVUs is coordinated with muscle fiber activation via ACh release at NMJs. The influence of motor unit recruitment on MVU perfusion is unknown. By defining the functional properties of motor units in the retractor muscle and observing MVU perfusion in response to motor unit stimulation, we will test the hypotheses that (i) each motor unit has fibers distributed throughout the muscle, that (ii) fibers within a given MVU derive from different motor units, and that (iii) motor unit recruitment is accompanied by MVU recruitment. Our long term goal is to elucidate physiological and pathophysiological interaction(s) among muscle fibers, motor nerves, and the local control of blood flow at the cellular level in vivo. We envision a new foundation from which to test hypotheses concerning the determinants of physical performance in healthy individuals as well as the mechanisms responsible for changes in work capacity with training, disuse or disease, and during the normal course of aging.